Pretreatment for Nerve Agent Exposure Chapter 6 PRETREATMENT FOR NERVE AGENT EXPOSURE MICHAEL A. DUNN, M.D., FACP*; BRENNIE E. HACKLEY, JR., PH.D.†; AND FREDERICK R. SIDELL, M.D.‡ INTRODUCTION AGING OF NERVE AGENT–BOUND ACETYLCHOLINESTERASE PYRIDOSTIGMINE, A PERIPHERALLY ACTING CARBAMATE COMPOUND Efficacy Safety Wartime Use Improved Delivery CENTRALLY ACTING NERVE AGENT PRETREATMENTS NEW DIRECTIONS: BIOTECHNOLOGICAL PRETREATMENTS SUMMARY *Colonel, Medical Corps, U.S. Army; Director, Clinical Consultation, Office of the Assistant Secretary of Defense (Health Affairs), Washing- ton, D.C. 20301-1200; formerly, Commander, U.S. Army Medical Research Institute of Chemical Defense, Aberdeen Proving Ground, Mary- land 21010-5425 †Scientific Advisor, U.S. Army Medical Research Institute of Chemical Defense, Aberdeen Proving Ground, Maryland 21010-5425 ‡Formerly, Chief, Chemical Casualty Care Office, and Director, Medical Management of Chemical Casualties Course, U.S. Army Medical Research Institute of Chemical Defense, Aberdeen Proving Ground, Maryland 21010-5425; currently, Chemical Casualty Consultant, 14 Brooks Road, Bel Air, Maryland 21014 181 Medical Aspects of Chemical and Biological Warfare INTRODUCTION Nerve agents are rapidly acting chemical com- cal as well and may impair physical and mental pounds that can cause respiratory arrest within performance. A pretreatment must be administered minutes of absorption. Their speed of action im- to an entire force under a nerve agent threat. Any poses a need for rapid and appropriate reaction by resulting performance decrement, even a compara- exposed soldiers, their buddies, or medics, who tively minor one, would make pretreatment use must administer antidotes quickly enough to save unacceptable in battlefield situations requiring lives. A medical defense against nerve agents that maximum alertness and performance for survival. depends completely on postexposure antidote treat- In the late 1980s, the United States, following the ment, however, has two key limitations: example of Great Britain, stocked the compound pyridostigmine for its combat units as a wartime • In the stress of a chemical environment, contingency pretreatment adjunct for nerve agent even well-trained military personnel will exposure.3 Several other Allies, including most not be uniformly successful in performing members of the North Atlantic Treaty Organization such tasks as self- and buddy-admin- (NATO), did so as well. At the recommended dose, istration of nerve agent antidotes.1 pyridostigmine is free of performance-limiting side • Aging, a change over time in the interac- effects. Unfortunately, pyridostigmine by itself is tion of nerve agents with the target enzyme ineffective as a pretreatment against subsequent acetylcholinesterase (AChE), renders oxime nerve agent exposure and thus it is not a true pre- therapy (an important component of nerve treatment compound. Pyridostigmine pretreatment agent antidotes) much less effective.2 As does provide greatly improved protection against explained below, aging poses an especially soman exposure, however, when combined with difficult problem for treating effects from postexposure antidote therapy. For this reason, the nerve agent soman. pyridostigmine is classified as a pretreatment adjunct. Research workers have attempted to develop true Because of these limitations of postexposure pro- nerve agent pretreatments whose own neurotoxic- tection, military physicians have focused on the ity is balanced or diminished by coadministration possibility of protecting soldiers from nerve agents of a pharmacological antagonist to their undesir- by medical prophylaxis, or pretreatment, designed able properties (eg, the carbamate compound phy- to limit the toxicity of a subsequent nerve agent ex- sostigmine, which is administered in combination posure. A significant problem with pretreatments, with a cholinolytic compound, such as scopola- however, has been their own potential for adverse mine). The potential and the problems of this pre- effects. In general, the pharmacological pretreatments treatment approach are considered in this chapter, that protect humans from the toxic effects of nerve along with a new pretreatment concept that in- agents are themselves neuroactive compounds. volves inactivating or binding nerve agents with Thus, their principal adverse actions are neurologi- scavenger macromolecules in the circulation. AGING OF NERVE AGENT–BOUND ACETYLCHOLINESTERASE Organophosphate nerve agents inhibit the active ylation of the AChE-bound nerve agent molecule pro- site of AChE, a key enzymatic regulator of cholin- ceeds depends on the nature of the nerve agent. ergic neurotransmission. As noted in Chapter 5, Table 6-1 shows the aging half-time of each of the Nerve Agents, agent-bound AChE can be reacti- five chemical compounds commonly considered to vated by a class of antidote compounds, the oximes, be nerve agents: tabun (GA), sarin (GB), soman which remove the nerve agent molecule from the (GD), GF, and VX. catalytic site of AChE. Aging is an irreversible reaction. After de- During the attachment of the agent with the en- alkylation, an AChE-bound nerve agent molecule zyme, a portion of the agent—the leaving group— can no longer be removed from the enzyme by breaks off. During a second, later reaction, one of an oxime. Thus, aging of enzyme-bound nerve the nerve agent’s alkyl groups leaves: this is the agent prevents oxime antidotes from reactivating process known as aging. The rate at which this dealk- AChE. This is an extremely difficult problem in the 182 Pretreatment for Nerve Agent Exposure TABLE 6-1 AGING HALF-TIME OF NERVE AGENTS Aging Nerve Agent RBC-ChE Source Half-Time GA (Tabun) Human (in vitro) >14 h1 Human (in vitro) 13.3 h2 RBC-ChE: erythrocyte cholinesterase 3 GB (Sarin) Human (in vivo) 5 h Data sources: (1) Mager PP. Multidimensional Pharmacochemistry. San Diego, Calif: Academic Press; 1984: 52–53. (2) Doctor BP, Human (in vitro) 3 h1 Blick DW, Caranto G, et al. Cholinesterases as scavengers for 4 organophosphorus compounds: Protection of primate perfor- GD (Soman) Marmoset (in vivo) 1.0 min mance against soman toxicity. Chem Biol Interact. 1993;87:285– Guinea pig (in vivo) 7.5 min4 293. (3) Sidell FR, Groff WA. The reactivatibility of cholinest- erase inhibited by VX and sarin in man. Toxicol Appl Pharm. Rat (in vivo) 8.6 min4 1974;27:241–252. (4) Talbot BG, Anderson DR, Harris LW, Yarbrough LW, Lennox WJ. A comparison of in vivo and in vitro 1 Human (in vitro) 2–6 min rates of aging of soman-inhibited erythrocyte acetylcholinest- erase in different animal species. Drug Chem Toxicol. 1988;11:289– GF Human (in vitro) 40 h1 305. (5) Hill DL, Thomas NC. Reactivation by 2-PAM Cl of Hu- man Red Blood Cell Cholinesterase Poisoned in vitro by Cyclohexyl- Human (in vitro) 7.5 h5 methylphosphonofluoridate (GF). Edgewood Arsenal, Md: Medi- cal Research Laboratory; 1969. Edgewood Arsenal Technical VX Human (in vivo) 48 h3 Report 43-13. case of poisoning with soman, which ages within 2 of exposure (mg•min). For example, a PR of 1.0 minutes. would indicate a completely ineffective antidote, Aging appears to be a key limiting factor in the because it means that the LD50 or LCt50 is the same efficacy of postexposure oxime therapy for soman for subjects who received an antidote and those who poisoning. One method for assessing the relative did not. A PR of 5, on the other hand, indicates that efficacy of antidotes and other countermeasures is the LD50 or LCt50 for subjects who received an an- the determination of their protective ratios. The tidote is 5-fold higher than that for subjects who protective ratio (PR) of an antidote is the factor by did not receive one. A PR of 5 or greater is consid- which it raises the LD50 or the LCt50 of a toxic nerve ered to represent a reasonable level of effectiveness agent challenge. Readers will remember that LD50 for medical countermeasures against nerve agents. is defined as the dose (D) of liquid or solid nerve This value was determined through threat analysis agent that is lethal (L) to 50% of the subjects ex- of battlefield conditions and consideration of the posed to it; LD50 is also described as the median fact that trained and equipped soldiers will be able lethal dose. LCt50 is the term used to describe the to achieve at least partial protection against nerve median lethal concentration for an aerosol or va- agent attacks by rapid donning of masks and use por agent, expressed as concentration (C) • time (t) of chemical protective clothing. PYRIDOSTIGMINE, A PERIPHERALLY ACTING CARBAMATE COMPOUND Pyridostigmine is one of a class of neuro- (CNS). Pyridostigmine has been used for many active compounds called carbamates. Its chemical years in the therapy of neurological disorders, structure and that of a related carbamate, physo- especially myasthenia gravis, a disease of neuro- stigmine, are shown below. Like the nerve agents, muscular transmission. In patients with myasthe- carbamates inhibit the enzymatic activity of AChE. nia gravis, inhibition of synaptic AChE is clinically As a quaternary amine, pyridostigmine is ionized beneficial. under normal physiological conditions and pen- As an inhibitor of AChE, pyridostigmine in large etrates poorly into the central nervous system doses mimics the peripheral toxic effects of the or- 183 Medical Aspects of Chemical and Biological Warfare H3C CH3 + N C O NCH3 H3C N C O H3C O H O NN CH3 CH3 Pyridostigmine Physostigmine ganophosphate nerve agents. At first it might seem fore